feat(schema): add pi-schema, JSON Schema builders and validation without eval

@earendil-works/pi-schema compiles JSON Schema (drafts 2020-12 and 7) once into
plain closures: no eval or new Function, so it runs under a strict CSP. It
passes 2,222 of the 2,223 core tests of the official JSON-Schema-Test-Suite,
checks Chord-style argument schemas within 1.2x of TypeBox's JIT, and keeps
3,200 compiled schemas in 7.9 MB (TypeBox: 54.7 MB).

- compile(schema, options) returns a Validator: check() is the boolean check
  (a type guard for builder schemas), errors() reports each failure with a
  JSON Pointer path, missing and unknown fields at the field.
- Builders (str, int, obj, opt, union, ...) return plain JSON Schema typed with
  the values it accepts (Static<typeof schema>).
- Formats are an optional entry point (./formats), adapted from TypeBox (MIT).
- Tests run the vendored suite (drafts 2020-12 and 7, with optional and format
  tests); npm run bench compares conformance, speed, setup cost, memory and
  bundle size with TypeBox, @cfworker/json-schema and omptype.

scripts/check-entry-graphs.mjs

@@ -26,6 +26,7 @@ const WORKSPACE = {
 	"@earendil-works/pi-codemode": "packages/codemode/src",
 	"@earendil-works/pi-telemetry": "packages/telemetry/src",
 	"@earendil-works/pi-mcp": "packages/mcp/src",
+	"@earendil-works/pi-schema": "packages/schema/src",
 	"@earendil-works/pi-tui": "packages/tui/src",
 };
 
@@ -34,6 +35,10 @@ const WORKSPACE = {
  * graph it is allowed to reach.
  */
 const BUDGETS = {
+	"packages/schema": {
+		// The format checks (vendored from TypeBox, about 40 KB) load only through ./formats.
+		".": { maxFiles: 10, forbid: ["schema/src/formats/"] },
+	},
 	"packages/ai": {
 		"./models": {
 			// Request telemetry reaches pi-ai's telemetry.ts and pi-telemetry's root (index, noop, metrics, memory).